A number of systems have been used or proposed which employ optical components, exclusively or partially, for communicating information (typically digitally), including systems for switching, routing, transmitting and the like. As one example, in the past, interferometers have been used to implement wavelength routers, for example as reported in B. B. Dingle and M. Izutsu, xe2x80x9cMultifunction Optical Filter with a Michelson-Gires-Tournois Interferometer for Wavelength-division-multiplexed Network System Applicationsxe2x80x9d, Optics Letters, volume 23, page 1099 (1998) and references therein. Generally, optical systems provide certain advantages over, e.g. fully-electronic networks (e.g. providing typically higher data rates, requiring less physical space, less susceptibility to electromagnetic interference, and the like) but also present their own set of issues. These issues include signal loss and signal dispersion, each of which can occur either during transmission along optical fiber cables (or other transmission lines) or in discrete equipment or components such as optical routers, switches, hubs, bridges, multiplexers and the like. Certain types of components, such as erbium doped fiber amplifiers (EDFA) can provide sufficient amplification to overcome some or all transmission line losses, thus providing a system in which the limiting factor tends to be dispersion.
In general, dispersion refers to change or degradation of the wave shape of an optical signal, such as an (ideally) square-edged pulse. In general, the fact that different wavelengths have different effective rates of transmission along an optical transmission line and/or different indices of refraction and reflection can lead to pulse (or other signal) degradation, e.g. such that an original signal comprising a sequential plurality of square-edged pulses will, as a result of so-called xe2x80x9cchromaticxe2x80x9d dispersion, be changed such that each pulse, rather than retaining a substantially square-edged shape will have a more rounded, Gaussian shape. Dispersion can lead to, e.g. partial overlap between successive pulses resulting in signal detection problems such as high bit error rates, decrease in spectral efficiency or other problems, especially when combined with signal loss (amplitude reduction). Accordingly, it would be useful to provide a method and apparatus for use in optical systems, which can compensate for and/or reduce the amount of dispersion effect.
The dispersion problems become even more severe for wavelength division multiplexing (WDM) systems. The dense wavelength-division multiplexing (DWDM) scheme is widely adapted as one of the optimal solutions to improve the bandwidth usage on optical fibers. By multiplexing multiple signals on different optical wavelengths, bandwidth of a single fiber can be multiple folded. Key optical components in DWDM systems include those which perform wavelength combining (multiplexing) and separating (demultiplexing) functions. The spectral response of the multiplexers and demultiplexers for DWDM applications are generally accompanied by certain dispersion effects that are determined by the underlying filtering technology. For example, the dispersion characteristic of a fiber Bragg grating can be determined by Hilbert transforming its transmission spectral response (e.g. as generally described in xe2x80x9cDispersion Properties of Optical Filters for WDM Systemsxe2x80x9d G. Lenz, B. J. Eggleton, C. R. Giles, C. K. Madsen, and R. E. Slusher, IEEE Journal of Quantum Electronics, Vol. 34, No 8 Page 1390-1402). The dispersion effects of wavelength multiplexing and filtering are very different from those of optical fibers. Optical fiber generally shows a linear dependency of its dispersion characteristic versus wavelength. Wavelength filters, multiplexers and demultiplexers, on the other hands, generally show nonlinear dispersion properties, e.g. correlated to its amplitude (spectral) response within its passband window.
A number of devices and techniques have been used or proposed in attempts to address dispersion issues. For example, a number of approaches have been based on using various bulk optics modules such as using dispersion compensation fiber (DCF), chirped gratings or other bulk optics modules. Previous approaches, however, have typically provided substantially static or unchangeable dispersion compensation parameters, such as providing substantially unchanging center wavelengths, dispersion slope and/or dispersion magnitude. As a result, such typical approaches lack versatility, making it difficult or substantially impossible to use a given module for more than one, or a relatively few applications. Accordingly, it would be useful to provide chromatic dispersion compensators which were adjustable, providing for adjustment or selection of one or more compensation parameters such as adjustment of center wavelength, adjustment of dispersion slope and/or adjustment of dispersion magnitude.
The present invention includes a recognition of the existence, nature and/or source of certain problems in previous approaches, including as described herein. In embodiments of the present invention, devices and techniques for dispersion compensation, preferably with adjustability of at least some parameters, are based on an optical interferometer. In at least one embodiment, a dispersion compensation apparatus provides an interferometer which splits an incoming optical signal into first and second distinct paths. The signal portions on the first and second distinct paths are later recombined and the interferometer outputs an output signal characterized by at least a first dispersion parameter. When the input signal includes an amount of dispersion, such as chromatic dispersion, the interferometer is configured (as described, e.g. more thoroughly below) such that the output signal has the dispersion reduced or substantially eliminated. Different embodiments or applications of the device and technique of the present invention may be used for compensating any of a variety of different magnitudes of dispersion that may occur in the input signal. For example, if the input signal has a first amount of chromatic dispersion, the interferometer preferably provides an amount of dispersion which is equal in magnitude but opposite in sign, to achieve an output signal with substantially zero dispersion. In this context, dispersion values are approximately equal in magnitude if the absolute value of the difference in magnitude is sufficiently small that, upon combining oppositely-signed signals, the resulting signal has a dispersion, in at least a first wave-length band of interest (such as a 90 to 95 percent transmission wavelength band) which is sufficiently low to achieve desired signal dispersion goals, such as being less than about 80 picoseconds nanometer (ps/nm) preferably less than about 20 ps/nm in the output signal passbands. In at least one embodiment of the present invention, the interferometer is adjustable e.g., so as to permit the magnitude of the compensating dispersion to be adjusted allowing a single device configuration to be used in any of a number of different applications.
In addition to (or in place of) providing for adjustment of dispersion magnitude, embodiments of the present invention can also be implemented so as to provide adjustment of dispersion parameters other than (or in place of) adjustment of dispersion magnitude. For example, embodiments of the present invention can be implemented to provide for adjustment of (compensating) dispersion center frequency and/or dispersion slope (i.e. the slope of dispersion expressed, e.g. as a function of frequency) and/or wave shape. In one embodiment, the magnitude of (compensating) dispersion is adjusted by adjusting the split-off ratio of an interferometer. In one embodiment, the xe2x80x9ccenterxe2x80x9d wavelengths (e.g. local dispersion maximum or minimum, e.g. with respect to pass bands or stop bands of an input signal) is adjusted by tuning the optical path lengths in the interferometer or components thereof. In embodiments in which the interferometer includes a resonator (e.g. Fabry-Perot cavity), dispersion parameters can be adjusted, e.g. by changing resonator mirror reflectivity, resonator path length and the like. The present invention can be used in connection with numerous types of interferometers including polarization interferometers, such as free-space interferometers, fiber optic interferometers, Mach-Zender interferometers and the like.
According to one aspect of the invention, an interferometer receives an input optical signal and outputs a signal after changing at least the dispersion of said signal. At least portions of the interferometer are adjustable to adjust at least a first dispersion parameter. Examples of dispersion parameters which are adjustable include dispersion magnitude, center wavelengths and waveshapes or slopes. Preferably the dispersion in the output signal is substantially reduced or substantially eliminated, compared to the dispersion of the input signal. By providing for adjustability of one or more dispersion parameters, a dispersion compensator can be appropriately adjusted for use in a variety of applications.